Erosion and plate tectonics have destroyed
most of Earth's craters. Fortunately, many ancient impacts
left behind distinctive changes in the rocks lying deep beneath
the crater. Like the scars from old wounds, these imprints
have remained long after the surface structures were eroded
away.

These signs of collision, and the few
actual craters that still exist, give us our closest look
at the impact process and shed light on how collisions have
shaped our own and other worlds.

This elliptical impact site in Sudbury, Ontario, is 1.9 billion
years old. Originally circular and much larger, the deformed
structure we see today consists of rocks that were deeply
buried at the time of collision. Mountain-building deformed
the structure. Later uplifting and erosion exposed these rocks.